The present invention relates to a method for the production of fine and ultrafine powders of various materials such as metals, alloys, ceramics, composites and the like with controlled physical properties. To carry out the method, a novel and flexible transferred arc plasma system providing the ability to control powder properties with a high production rate has been developed. The transferred arc plasma system comprises a transferred arc plasma reactor and a separate quench system within which powder condensation occurs.
Fine powders of metals, alloys, ceramics, composites and the like have a wide variety of applications in various fields such as aeronautics, electronics, microelectronics, ceramics and medicine. Currently, generation of fine powder, i.e., powders having an average particle size between 0.1 and 10 xcexcm, is mainly accomplished via 3 different techniques: 1) hydrometallurgy, 2) spray pyrolysis and 3) milling. Among the disadvantages of the above techniques are high operating costs, production of non-spherical particles and generation of toxic or difficult to handle by-products.
The benefits obtained with ultrafine powders, i.e., powders with an average particle size lower than 100 nm, are mainly due to their small particle size, which results in a higher surface area/volume ratio. Consequently, ultrafine powders may have advantages over fine powders when used in the above fields.
The preferred methods for the production of fine powders are hydrometallurgy and spray pyrolysis. However, these methods have several major drawbacks including preparation and handling of the feed materials like chlorides and nitrates, which are very often toxic and difficult to handle, environmental emission control requirements for gaseous and liquid effluents, and a difficulty to produce average particle sizes below 100 nm.
Thermal plasma based vapor condensation methods have demonstrated their ability to generate average particle sizes below 100 nm without the handling and environmental problems associated with hydrometallurgical and spray pyrolysis methods. These problems are avoided because the feed materials are generally inert. Examples of such materials include pure metals, alloys, oxides, carbonates etc. Such plasma methods are able to vaporize or decompose these feed materials because of the high-energy input that can be achieved.
Thermal plasma generation is typically accomplished via 2 methods, i.e., high intensity DC arcs which uses currents higher than 50 A and pressures higher than 10 kPa, or high frequency discharges such as an RF plasma. Because of their high-energy efficiency, DC arcs are generally preferred. DC arcs are classified as transferred when one of the electrodes is a material being processed, and non-transferred when the electrodes are non-consumable. Since transferred arc systems pass electrical current directly through a material being processed, their energy efficiency is higher than non-transferred transferred arc systems. Because of the extremely high heat input into the material acting as the electrode, vaporization or decomposition occurs, thus producing a vapor phase that is then cooled to induce the formation of the powder. The powder product is then typically recovered in a filtration unit.
Thermal plasma based vapor condensation methods which utilize a transferred arc have not been successful up to now to generate fine or ultrafine powders of materials like metals, alloys, ceramics or composites on a commercial scale because of their low energy efficiency, low production rate, poor yield, and rudimentary control of powder properties such as particle size and distribution, shape, and crystallinity. In addition, this method is typically used for the production of powders with an average particle size lower than 0.1 xcexcm, which has also contributed to its lack of success on an industrial scale because today""s market requires powders with larger particle sizes.
In addition to producing fine and ultrafine powders of various pure materials, transferred arc plasma systems can also be used for the production of fine and ultrafine powders resulting from the interaction of two or more components (chemical reaction) or elements (alloying).
Although transferred arc plasma systems can operate batchwise, it is preferred that they be operated in a continuous manner. The material to be vaporized or decomposed can be fed continuously in the reactor in several manners. For example, it can be fed into a crucible either from the top thereof by a side tube in the reactor wall. The material can also be pushed upward underneath the plasma in a continuous manner, or fed directly into the plasma torch. Depending on the powder to be produced, the operator will select the appropriate method. Generally, the preferred feeding method is through one or more tubes located in the upper portion of the reactor. The feed materials can be in solid (wire, rod, bar, chunks, shots etc.) or liquid form. When in liquid form, the feed material can also be pumped into the reactor.
U.S. Pat. No. 4,376,740 discloses a method for producing fine metal powders which involves reacting a molten metal or alloy with hydrogen using an arc or plasma discharge, or an infrared radiation which dissolves the hydrogen in the metal. When the dissolved hydrogen is released from the molten metal, fine metal powders are generated. Using this method, a low production rate and yield is attained because of the use of a cold-walled reactor and a water-cooled copper mold which is used to support the material being processed. The maximum production rate reported is less than 240 g/hr. Further, there is no mention or suggestion of control of powder properties.
A critical aspect of transferred arc plasma systems is that they consume a lot of energy. It is therefore imperative to maximize its efficiency to have a viable commercial method. This means that the temperature within the reactor must be maintained as high as possible to prevent condensation of the vaporized or decomposed materials therein, either on the plasma chamber walls, outside surface of the plasma torch or the mold, which is very often a crucible. Such maximization would obviously result in higher production yields of powders. Because of the extreme conditions prevailing in the transferred arc reactor, many elements are generally water-cooled to extend their operating life. Obviously, such cooling has the effect of reducing the energy efficiency of the method. It has been proposed in Ageorges et al. in Plasma Chem. and Plasma Processing, 1993, 13 (4) 613-632 to modify the interior of a transferred arc reactor by covering its internal surfaces with a graphite lining to retain as much heat as possible inside the reactor.
Ageorges et al. supra, also disclose the production of ultrafine aluminium nitride (AlN) powder using a transferred arc thermal plasma based vapor condensation method. Vaporizing aluminium and reacting it with nitrogen and ammonia in an insulated plasma chamber produces the desired aluminium nitride product. Aluminium is vaporized by using it as the anode material in a transferred arc configuration that employs a thoriated tungsten tip cathode. The aluminium being vaporized is in the form of an ingot placed in a graphite crucible surrounded by a water-cooled stainless steel support. Because of the presence of that water-cooled jacket, the energy efficiency of vaporization is reduced. A disadvantage of this process is due to the fact that the formation of powder occurs in the plasma chamber because of the injection of reactive gases in the plasma chamber, i.e., nitrogen and ammonia. Ageorges et al. specifically state that the plasma chamber is xe2x80x9cfilled with fume products which recirculate in the furnacexe2x80x9d. As a result, powder property control is very crude because of the difficulty in properly controlling nucleation and growth of the powder product in the plasma chamber. The particles produced are reported to have a nominal particle size of 135 nm based on specific surface area measurements.
To better control the formation of ultrafine aluminium nitride powder, Moura et al. in J. Am. Ceramic Soc., 1997, 80 (9), 2425-2428 propose the separation of aluminium vaporization and aluminium nitride formation. This is accomplished by vaporizing an aluminium anode in a transferred arc reactor in which no reactive gas is introduced, and reacting the aluminium vapor with ammonia injected at a single point in a separate reactor tube attached to the exit of the plasma chamber. The aluminium nitride powders generated with this method have a mean particle size of approximately 20 nm.
Da Cruz et al. in IEEE Trans. on Plasma Science, 1997, 25 (5), 1008-1016 reports using a thermal plasma based vapor condensation method using a DC transferred arc plasma system. In this work, an aluminium anode is vaporized by striking a thermal Ar or Ar/H2 arc to it. The aluminium vapor is reacted and cooled rapidly in a separate quench tube to generate ultrafine aluminium nitride powders. The reactor exit gas containing the aluminium vapor is quenched at a single point using an Ar/NH3 mixture resulting in the production of ultrafine powders. This technique is similar to that described by Moura et al. supra. The powders produced have very high specific surface area (40-280 m2/g) and an average particle size of less than 50 nm.
Chang et al. in Third Euro-Ceramics, 1993, 1, 15-20 use a transferred arc thermal plasma based vapor condensation method to produce ultrafine ceramic and composite powders. In their production of SnO2 or Ag/SnO2 powders, a tin or silver/tin anode is vaporized by striking an arc to it while it is contained in a graphite crucible surrounded by a water-cooled stainless steel support similar to that described by Ageorges et al. supra. A reactive gas, i.e., oxygen, is added to the plasma chamber, resulting in the formation of the product that is then transported to a quenching section. Because oxygen is added into the plasma chamber, both vaporization and reactive steps are conducted in one vessel. In the works of Da Cruz et al. and Moura et al. supra, the vaporization and reactive steps in the production of the powder compound are separated to better control the particle formation process.
Chang et al. in 12th International Symposium on Plasma Chemistry, 1995, 1207-1212 use a similar method to that of Chang et al. in Third Euro-Ceramics supra. In this work, silica powders are produced. The silica raw material is vaporized by injecting it in a particulate form, i.e., sand with a particle size of 100-315 xcexcm, into the arc. The arc is struck between a non-consumable cathode and an anode that is not made of the material being vaporized. As a result, the energy efficiency of this method is likely to be lower than those previously mentioned which use true transferred arc operation. Most of the particles that made up the silica powder product had a particle size ranging from 50 to 400 nm.
It has been shown theoretically that by controlling the initial vapor concentration and temperature, residence time of particle nucleation and growth, and cooling profile, one may have some control on the particle size and distribution and crystallinity. This is shown by Okuyama et al. in AIChE Journal, 1986, 32 (12), 2010-2019 and Girshick et al. in Plasma Chem. and Plasma Processing, 1989, 9 (3), 355-369. When this method is used for fine powder production as demonstrated by Ageorges et al. and Chang et al. supra, control of the powder properties is very crude because no apparatus or procedure is described to accurately control the nucleation, growth and crystallization of particles in the quench section. In addition, in both the works of Ageorges et al. and Chang et al. supra, no attempt is made to limit the nucleation and growth of particles in the plasma chamber, which also contributes to the lack of proper control of powder properties. Control of the particle size and distribution, and crystallinity of fine and ultrafine powders produced using a transferred arc thermal plasma based vapor condensation method is therefore very limited.
In certain fields, such as electronics or metallurgy, mean size and distribution, and crystallinity of the powder represent critical properties. Accordingly, if such properties can be controlled during the manufacturing process of the powders, it would give to its producer a significant advantage over current fine and ultrafine powder manufacturers.
In accordance with the present invention, there is now provided a transferred arc thermal plasma based vapor condensation method for the production of fine and ultrafine powders of materials such as metals, alloys, ceramics, composites, and the like. More specifically, the method comprises the steps of:
providing a material to be vaporized or decomposed in a plasma reactor;
striking an arc between the material and an electrode to generate a plasma having a temperature sufficiently high to vaporize or decompose the material and form a vapor thereof;
optionally injecting a diluting gas in the plasma reactor;
transporting the vapor by means of the plasma gas and optional diluting gas into a quench tube wherein the vapor is condensed and powder formation occurs, the quench tube comprising
a first section for indirectly cooling or heating the vapor and any particle present therein, to substantially control particle growth and crystallization; and
a second section coupled to the first section for directly cooling the vapor and any particle present therein; and
collecting and optionally filtering the powder particles in a collection unit.
In a preferred embodiment, the diluting gas is heated to a temperature corresponding to that of the vapor, or at least 1000 K, before being injected continuously or semi-continuously in the plasma chamber. The injection flow rate of the diluting gas can be varied depending on several parameters such as production rate, powder properties, plasma gas flow rate, vapor concentration etc. Any operator skilled in the art can determine the optimum diluting gas injection flow rate.
In a further preferred embodiment of the present method, a straight polarity configuration is used, i.e., the liquid material in the crucible is the anode and the electrode is the cathode. In addition, the electrode is non-consumable and is located inside the plasma torch.
In a second aspect of the present invention, there is provided a quench tube suitable for the condensation of vapor such as that produced from a transferred arc reactor. More specifically, the quench tube comprises a first section with an elongated substantially tubular body having cooling or heating means around the body for indirectly cooling or heating the vaporized material passing therethrough, thus controlling the growth and crystallization of the particles; and a second section coupled to the first section comprising means for directly cooling the vapor and particles thereof
In a preferred embodiment, the second section comprises an extension of the tubular body of the first section, and the direct cooling is done by injecting a cooling fluid directly onto the vapor.
The inner tube diameter and the length of the first section of the quench tube can be varied depending on various parameters, such as powders to be produced, properties desired for these powders, flow rate of the carrier gas, particle size desired, etc. Any experienced engineer or operator skilled in the art may adjust these parameters according to powder properties desired.